Leucyl‐tRNA Synthetase

Two forms of leucyl‐tRNA synthetase of Escherichia coli B were separated and purified about 850‐fold to apparent homogeneity. Their molecular weights were identical, 104000, as determined by Sephadex filtration and polyacrylamide electrophoresis. One of these forms catalyzed the leucine‐dependent ATP‐PP i exchange reaction but was unable to transfer leucine to tRNA even though it could form a complex with tRNA Leu . The other form was active for both ATP‐PP i exchange and charge of tRNA. Evidence was obtained for the interconversion between these two forms of leucyl‐tRNA synthetase and for the existence in the E. coli 105000X g supernatant of a factor stimulating this interconversion; during the incubation of one of these forms with the supernatant, the other form appeared and an apparent equilibrium between the two forms was observed.